1. Field of the Invention
This invention relates to methods and apparatus for testing integrated circuits and more particularly to such methods and apparatus which are used for automated testing of a plurality of integrated circuit chips mounted on an elongate flexible strip similar to motion picture film.
2. Description of the Related Art
One method of manufacture and testing of integrated circuits includes a process known as tape automated bonding. This process utilizes an elongate flexible strip similar to standard motion picture film upon which is deposited a thin layer of conductive material such as copper. This layer is photoetched to form a plurality of sequential lead frames which are substantially identical to one another. Next, an integrated circuit chip or die is bonded to the leads of each frame.
The strip typically includes a number of sprocket holes along each side thereof for engagement with rotating sprocket wheels which pull the strip, and the lead frames and associated die thereon, through different stages of the manufacture and testing process. Like motion picture film, the strip having the lead frames and die mounted thereon is wound onto and off of reels during the manufacturing and testing process. When the process is complete, the strip is cut into segments, each of which includes a lead frame and the die associated therewith, which are packaged to form the finished product.
The die are tested while on the strip to determine whether they meet operating specifications and to confirm that the electrical connections between the leads and the die are properly made. Such testing is accomplished by means of a probe test card which includes a plurality of probes extending downwardly therefrom. Each of the probes comprises a relatively fine wire against which a downward biasing force is applied.
In one configuration for testing, the strip, with the die directed upwardly, is pulled by the sprockets beneath the probe test card. The testing equipment periodically stops strip movement so that each die is temporarily positioned beneath the probe test card. When so positioned, the card lowers until each of the probes are biased against a test pad formed on the copper leads of the frame beneath the card. Thereafter, voltage is applied and measured via the probes to confirm that the die meets operating specifications and that the lead connections to the die are properly made. After so doing, the probe test card raises and the strip advances until the next die in sequence has stopped beneath the probe test card. The card again lowers to test the next adjacent die and lead connections as described above. The foregoing process is repeated until each of the die on the strip is tested.
The foregoing prior art method and apparatus suffers from several disadvantages. As noted above, the probes on the card are very fine and, depending upon the integrated circuit being tested, may number over 100 with relatively small spacing between probes. If the probe is bent so that the tip thereof is displaced a distance equal to the thickness of a human hair, the probe may not strike the test pad on the lead frame with which it is designed to make electrical contact. Under such circumstances, it is likely that testing will indicate a defective die and/or lead connection. Since the testing machine is equipped with a punch for excising defective circuits from the strip, a number of good die which are properly connected to their associated leads may be punched from the strip.
Probes are typically bent in one of two ways. First, as the strip starts and stops under control of the testing equipment, a misfeed may cause a slight buckle which elevates a section of the strip having test pads thereon. When the probe test card lowers, probes over the buckled strip are jammed into the strip, thus deflecting the same. Also, if even a small amount of strip movement should occur with probes contacting test pads, probes are likely to bend.
The automated testing process typically is stopped every one to two hours in order to correct damage to probes on the card.